Pull-off device for telemeter indicators



F. R. SIAS mmh 2s, 1944.

PULL-OFF DEVICE FOR TELEMETER INDICATORS Filed Jan. 14, 1943 )Va/wf HisAttorney.

March 28, 1944. F` R slAs 2,345,011

PULL-OFF DEVICE FOR 'IELEMETER INDICATORS Filed Jan. 14, 1943 2Sheets-Sheet 2 Inventor' F'r'eder'ck R. Sias,

.KIDS

r-Is Attor-he.

y purpose is described" Patented Mar. 28, 1944 rum-oFF DEVICE FoaTELEME'IEB INDICATORS Frederick It. sia, Marblehead, Mass., assigner toGeneral Electric Company, a corporation of New 4York Application January14, 1943, Serial No. 4172,31'14 6 Claims. .(Cl. 171-95) My inventionrelates to current responsive device's and measuring instruments; Itconcerns particularly ratio instruments and receivers or indicators fordirect current telemeterng systems.

It is an object of my invention to provide improved pull-oi arrangementsfor pulling the pointer or the movable index or target of a receiver orindicator to 'an oit-scale position in case of power failure.

A further object of my invention is to provide nearly uniform pull-oliforce` throughout a large scale angle, and to provide a pull-offarrangement suitalle for use with a long scale in-v strument which doesnot aiect the shape of the instrument calibration curve and which doesnot require strong pull-off torque in order to be effective.

Still another object is to decrease voltageand temperature errors ininstruments having pulloiI arrangements.

Other and further become apparent as proceeds.

The present application is a continuation-inpart of my copendingapplications, led November 9, 1942, Serial No. 465,081; illed November9, 1942, Serial No. 465,079; iiled November 21, 1942.,

objects and advantages will the following description Serial110,466,436; all assigned to the same asl sig-nee as the presentapplication.

instruments, particularly receivers or indicators for telemeteringsystems, it is delsirable to provide an arrangement for moving thepointer or index oil' scale in the case of current energizing thetelemetering system in order to guard against false readings or indica-4tions. One form of pull-011` arrangement for this 'in Patent No.2,181,803, Fans. The Faus patent relate: to'an indicator of the movingmagnet type in which tli'e rotorconsists of a permanent magnet, A pull-omagnet consisting of a permanently inagnetized rod is mounted at oneside oi the axis of rotation of the rotor in such an angular position asto set up a magnetic iield which deiiects the rotor to an olf-scaleposition, that is, a position inwhich the pointer or index lies beyondthe end, of the scale to show that the system is not properly energizedand to prevent false readings or indications. It will be understood thatthe torque produced'by the reaction between the pull-oil.' magnetand therotor is made less than that provided by the normal instrument eld whichdeilects the rotor accords ing to the indications to be transmitted bythe of failure of a source 30 In certain types of indicating andmeasuringtelemetering system when the system iis properly energized.When the system is energized the pull-oil torque has lrelatively littleeiect on the c lindrical shield.

The pull-ofi magnet exerts a torque on the moving magnet or rotor oi theindicator and this torque varies substantially sinusoidally with theangular position of the rotor so that the eifectof the pull-oil! fieldon the instrument calibration during normal operatipn of the instrumentis different at different angular positions of the rotor.

.In order to make the pull-oil.' field substantially uniform throughoutthe length of the instrument. scale and thereby avoid distortion of theinstrument scale, I provide a modiiied pull-ofi` arrange' ment. Incarrying out my invention in its pre. ferred form I combine with thepermanently magnetized pull-oi magnet a second pull-oi element which iscomposed of magnetizable material which does. not tend to -becomepermanently magnetized but in which a magnetic flux is readily inducedby the magnetic eld. The second pull-,off element is so mounted inrelation to the moving magv net of the instrumentthat the said secondelement also tends to deect the moving magnet to an olf-scale position.The relationship -be- 5 tween the two pull-oil? elements is Inaiile suchthat,throughout the scalerange of the instrument, variations in thepull-oil? torque from one pull-oil.

element tend to compensate for variations in pulloi torque of'the..otherpull-,oil element as the angular position of the moving magnet isvaried.

A .better understanding oi the invention will be. aiorded by thefollowing detailed description ccnsidered in connection with theccompanying drawings, `and those features of the invention which arebelievedto be novel and patentable will be pointed out in the claimsappended hereto.

In the drawings Fig. 1 is a plan view of a direct current .telemeterindicator or receiver embodying my invention; Fig. 2 is a view of asection cut through the apparatus of Fig. 1 by a plane 2, 2' indicatedin Fig. .1; Fig. 3 is'a fragmentary view ofthe apparatus of Fig. 1 withthe cover and certain other parts removed to expose the interior;

erably rotatable,

' direction parallel to the physical Fig. 4 is a view through the coveror subbridge of the apparatus of Figs. 1 and 2 cut' by a plane 5,indicated in Fig. 1, so as to show the manner of mounting the pull-oimagnet; Fig. 5 is a diagrammatic view of another embodiment of myinvention illustrating my pull-off arrangement used in a modied form ofreceiver or indicator; Fig. 6 is a diagram explaining the-principle ofoperation of a permanently magnetized pull-off magnet employed alone;Fig. '7 is a diagram explaining the manner of operation of vamagnetizable but not permanently magnetized pull-off element used alone;Fig. 8 is a diagram explaining the eifect 'of combining the pull-offmembers of Figs. 6 and '7, and Fig. 9 is a set of curves explaininggraphically the operation of the pull-oii units as showndiagrammatically in Figs. 6, 7 and 8. Like reference characters are usedthroughout the drawings to designate like parts.

It will be understood that in ratio instruments indicatype a movable,prefmagnet or rotor, is actuated by the magnetic field produced by astationary field structure. The eld structure ordinarily of electricalwindings of two or more circuits comprising electrical coils producingmagnetic fields, the resultant of Which-depends upon the relativemagnitudes of electrical currents in the and direct current telemeterreceivers or tors of the moving magnet instrument circuits so thatvariations in the circuit produce variations in the angular position ofa resultant magnetic field, thereby causing deflection or rotation oivthe moving magnet or rotor. In the case of a measuring instrument therotor carries a pointer and there is a graduated scale .cooperating withthe pointer. My present invention is not directly concerned with thesefeatures of the instrument or indicator per se, and I shallA thereforerst describe the operation of the pulloi! arrangement without referenceto the magnetic field produced by the field structure of the instrument.l

In Fig. 6 there is shown a permanent magnet "I I rotatable about acenter point I2 with a line of polarization NS transverse to the axis ofrotation o! the magnet II, which` thus forms the rotor of an instrumentor indicator, the eld structure of which is not shown. For deilectingthe rotor I I to an off-scale position when the eld structure is notenergized, a permanent magnet I3 may be provided which is mounted in axed position sov as to -form a pull-off magnet. For convenience thepull-on magnet I3 is shown in Fig. 6 as having its physical axisintersecting the axis of rotation I2 of the rotor II, but it isunderstood that Fig. 6 is merely diagrammatic and does not necessarilyrepresent the actual mounting position of the pull-off magnet in actualapparatus.

The diagrammatically represented pull-off magnet I3 in Fig. 6 is to beconsidered as representing permanent magnet pull-ofi' ilux having a axisof the mag'- net I3. Thus when'the rotor I I is in the position shown inFig. 6 its line of polarization NS is parallel to the pull-on fieldproduced by the pull-off magnet I3, and in the same sense, so that thereis no tendency for the rotor II to deflect. For

convenience in designating various angular positions of the rotor Il apointer I4 is shown diagrammatically. The pointer I4 is in a positionmarked as an 180 degree position in Fig. 6. If the rotor I I should bein any other position than that shown in Fig. 6, the pull-off eld tendsto return it to the position there shown. The pulloi'! torque isgreatest when the magnet is in either position 90 degrees from theposition shown in consists right-hand side of the circle.

-A grees from lill Fig. 6, that is, with the pointer I4 in at either thedegree or the 270 degree position, since in this case the magneticmomuit is greatest. This is represented in Fig. 6 by the numeral 2approximately representing the pull-off torque in the 90 degree and the270 degree positions. The numeral I is intended to represent that thepull-off torque is about half as great in the' 45, 135, 225 and 315degree positions. However, for a magnet rotating in a perfectly uniformpull-off eld the pull-off torque would theoretically vary sinusoidallywith the angular position of the rotor. As represented by the arcuatearrows I5 the pullgree positions. This ate positions between zero and180 degrees at the As represented by arcuate arrows I6 the pull-offtorque is counterclockwise in the rotor positions in the other half ofthe circle. Thus the pull-off torque from the permanent magnet I3 variesbetween a maximum in one direction and a maximum in the oppositedirection and has two opposite peak values as the rotor is rotated 360degrees.

On the other hand, if the permanent magnet I3 were replaced by a pieceof magnetic material I'I represented in Fig. 'I which is magnetizablebut does not become a permanent magnet, that is, has relatively highpermeability and low coercive force, the pull-oir torque would have twomaximum 'values and two minimum values. throughout the 360 degreepositionvof the rotor II. Thus, the pull-off torque will be zero asindicated by the numerals zero in Fig. '7 when the pointer I4 is eitherin the 180 degree or the zero degree position because in either case theline of magnetization NS of the rotor II is parallel with the magneticfield which the rotor II induces in the soft iron piece II. In addition,there are unstable positions at 90 degrees and 270 degrees in which thepull-ofi?` torque is also zero. It has maximum values indicated by thenumeral l placed at the four positions which are 45 de- A the zerotorque positions. As represented by the arcuate arrows I8, I9, 20 and 2lthe pull-off torque reverses from counterclockthe degree position;counterclockwise at the -225 degree position, and clockwise at the 315degree position. The effect of combining the pull-oi arrangements orFigs. A6 and '7 is represented in Fig. 8 where it is assumed that thedimensions and spacing of the elements and the strengths or the magnetsare such that the maximum torque .produced by the soft'iron piece I'I ishalf that produced by the permanent magnet I3. This is represented inFigs. 6, 7 and 8 by the numerals I and 2 at the various angularpositions representing the relative values of pull-off torque. In Fig. 8furthermore the soft iron.piece` I'I has been mounted 45 degreescounterclockwise from the permanent magnet I3 so that it will tend todeflect the rotor II further counterclockwise, that is, further beyondthe zero end of the scale for a reason which will become apparenthereinafter. In Fig. 8 the inner circle oi.' arcuate arrows 22represents the pull-off torques produced by the permanent magnet I3 asshown in Fig. 6. The next circle of arcuate arrows 23 represents thepull-oil torques produced by the soft iron piece I1; and the outercircle of arcuate arrows 24 represents the combined pull-off torques. Itwill be observed that the circle of arrows 22 corresponds to the arrowsshown in Fig. 6. The

circle of arrows 23 corresponds 'to arrows shown in Fig. 7 except forrotation of the entire figure 45 degrees counterciockwise.

By adding the torque values represented by= r.in the outer circle 24.Thus in the zero degree position the permanent magnet torque being zeroand the soft iron -torque having the value of 1,

the resultant torque is the value of 1 in the counterclockwisedirection. On the other hand at the 45 degree position where thepermanent magnet pull-off torque has the value of 1 and the soft ironpull-oif torque has the value of zero, the resultant torque has thevalue of 1 in the clockwise direction. The turques at these two pointsare shown by the directions of the arrows 25 and 26, respectively. Atthe 90 degree position the torques from the two pull-olf elements arecumulative' and give a total`.resu1tant torque of 3 in the clockwisedirection. On'the other hand, in all the other positions diifering fromeach other by 45 degrees, the resultant torque has theulform value of 1.Thus at the 270 degree position the permanent magnet torque which hasthe torque produced by the soft iron element I l, the peak value of thecurve 3l being xa of the peak value of the curve 30. 'I'he curve 29 isthe resultant of the two curyes 3l)A and 3|. In all three lcurves thevalues of torque are vplotted in a. vertical direction, and the angularpositions of the pointer I4 are represented by the distances in thehorizontal direction.

When the pull-off arrangement represented diagrammaticaily in Fig. 8V isemployed, the instrument scale is mounted in such a position that thepointer Il sweeps the scale while the rotor is within the regionrepresented by the 225- degree arc 21. For example, a graduated scale 32may be mounted in the position shown in Fig. 8 with respect to thepull-off elements, the rotor and the pointer. The scale 32 may have ananguiar length from 120 to 150 degrees. In the most commonly used longscale telemeter receivers one' end of the scale, usually thecounterclockwise end 33, is referred to as the zero end voi the scale,and the other end 34 is referred to as the full scale end. It will beunderstood-however, that in some c'ascs theindicator will be used forthe measurement or indication of quantitiesl which :would call forplacing a zero division ,atthe center r some other portion of the scale.

value of 2 in the counterclockwise position is opposed by the soft irontorque having the value of 1 in the clockwise directionl and theresultant torque has the value of 1 in the counterclockwise direction.Similarly all other positions from v18() degrees to 360 (or zero)degrees on the left-hand side ofthe circle have the value of 1 in thecounterclockwise direction. At the 22l/2 degree and the 1571/2A degreepositions .theresultant torque falls to zero and between the 22% degreeand 157% degree positions on the right-hand side of the circle theturques have the reverse direction, that is, clockwise wise.

Although Fig. 'isdiagrammatic and does not take into consideration minorvariations at intermediate positions, it indicates that throughout an'angle of 225 degrees represented by the double ended arcuate arrow "21there is no reversal in the pull-ofitorque and this Vtorque remainssubstantially uniform throughout an angle of `180 degrees represented bythe left-hand side of the circle. If intermediate positions were takeninto consideration, some fluctuation in pull-off torque would beobserved. However,` the degree of uniformity of pull-of torque may beimproved at a slight expense of the length of scale angle overwhichthetorque is uniform by making the maximum pull-off torque from thesoft iron element I1 a little less thanone-half the maximum pull-offtorque from the permanent magnet I3.A Where the instrument is to have ascale length lof approximately 120 degrees.

iron pull-off torque rs of the permanent magnet pull-off torque so thatthe resultant pull-off torque is about 116 of the maximum pull-olftorque instead of counterclock- `I consider it most satisfactory to makethe soft range of approximately 120 degrees the resultant torquerepresented by the substantially dat portion 28 of the curve 29 is .verynearly uniform. In Fig. 9 the curve 30 is a fundamental frequency sinewave representing values of torque-producedv by the permanent magnet I3and the curve 3| is a double frequency sine wave representing theInusing the expression zero end of the scale in -the description andclaims therefor, I do not mean to limit the invention to any speciiictype of scale calibration but I mean to include any scale or set ofindices having an endpoint beyond whicha pointer .or target carried bythe movable member is carried when the source o power energizing theinstrument fails.

The combination permanent magnet and soft iron pull-off torquearrangement represented by Fig. A8 maytake various practical forms someof which will be described more in detail in connection with directcurrenttelemeter receivers of the types disclosed in my aforesaidcopending applications, Serial No. 465,079 and Serial No. 465,081. Forexample, one way of carrying out my invention is to provide apermanently magnetizedpulloffv magnet I3 and one or two soft ironpull-ofi' elements in the form of'soft iron screws 35 and 38 cooperatingwith apermanent magnet rotor II as shown in Fig. 3. It will beunderstood that, for adjustment of the Vrelative pull-on effects andcorrection of manufacturing irregularities, the permanent magnet elementI3 and the soft iron elements 35 and 36 may eachv be `adjustably mountedwith provisions for varying the distance may be adjustably mounted (byconventional means not shown in Fig. 3).v The soft iron elements 35 and36 may be threaded in to a rotatable carrier 3l to provide for angularadjustment and as shown may be threaded into or out of the carrier 31 toprovide strength adjustment.

Ordinarily adjustment of the permanent magnet I3 is not necessary afterthe design of the instrument has been determined and the magnet has beenmagnetzed and aged to give it the strength desired for the type ofinstrument in which it is employed. Accordingly the magnet I3 may bemounted in-a iixed positicnas illustrated in F'g. 1 which will beexplained more in detail hereinafter.

- Figs. 1, 2, 3 and 4 illustrate the manner of employing the combinationof permanent magnet -and soft iron pull-oil` in a temperature indicatorof a type described in my aforesaid patent application, Serial No.465,079. This type of-temperature indicator comprises a bridge circuit(not shown in thel present application) and a cross coil instrumentcorresponding to a ratio instrument having two 'circuitsand which isresponsive to the relative currents in the two circuits.

The instrument illustrated comprises an insulated base and terminalblock 38 (Figs. 1 and 2), a frame 39 secured to the terminal block 38, asub-bridge and cover 40 secured to the top of the frame 39, a top bridge4I secured to posts 42 protruding through the sub-bridge40, a magneticshield 31 rotatably mounted within the frame 39, the magnetic rotor I Icarried by a shaft 43 rotatably mounted within the frame 39substantially concentric with the magnetic shield A 31 and a stationarymagnetic eld structure for. i producing vmagnetic elds varying in aresultant angular direction to produce rotation of the rotor II inaccordance with variations in an indication or measurement to beproduced.

The stationary field structure comprises a main winding and an auxiliarywinding. The former consists-of four coils 44, 45, 46 and 41 (Fig. 3)

electrically connected in series and wound around `a hollow winding form48 with a common magnetic,axis transverse to and substantiallyintersecting the rotor spindle 43. The auxiliary or crosscoil windingconsists of a pair of coils 4,9 and 50 electrically connected in seriesand also having a common magnetic axis transverse to and substantiallyintersecting the rotor spindle 43, but substantially perpendicular tothe magnetic axis of the main winding consisting of the coils 44l to 41.The electrical eld windings and the winding form 48 are f such shape tocause the space within the magnetic shield 31 to be substantiallyfilled. The .winding form 48 is of split construction to facilitateassembly and has a substantially cylindrical opening I therein toreceive the rotor II. A damping cup may be provided consisting of asplit hollow metallic cylinder 48' at the interiorof the winding form48. The element 31 previously referred to as a carrier for the screws 35and 36 in connection with Fig. 3 is composed of Y magnetically permeablematerial so as to serve as a magnetic shield for the instrument.

'I'he rotor II, as represented in Fig. 3 by dotted.

lines, is in the shape of a right circular cylinder with flattenedsides. Suitable openings-are left in the bottom and the top of the mainwinding and the winding form -4I! to receive the rotor spindle 43.4Likewise an opening 52 is provided in the sub-bridge or cover 4II forthe upper end of the rotor spindle 43.

The pull-oil magnet I3 may consist of a bar of high coercive force lowpermeability magnetic.

material secured to the sub-bridge 40, which in this case is composed ofnon-magnetic material. For example, the opening 52 may be extended toform slots 53 of suitable shape to receive the bar .magnet I3, and theedges of the slot 53 may be peened over at the points 64 to hold thepull-ofi magnet I3 in place', as illustrated in Figs. 1 and 4.

It will be apparent that the relative magnitudes of the electricalcurrents flowing in the main winding consisting of the coils 44, 45, 46,and 41 and the auxiliary windings consisting of the coils 49 and 50 willdetermine the resultant angular direction of the magnettofi'leldproduced by the field windings. This i turn will determine the angularposition of the rotor II and the pointer I4 with respect tothe scale 32so long as the point 33 of the scale 32.

'The represented position of the magnet I3 in Fig. 3 does not correspondstrictly with that shown in Fig. 1 as its position has been moved topermit more clearly representing the rotor I I in l5 Fig. 3.

It will be observed that in the arrangement of Figs. 1 to 4 the angulardirection ofthe'mid scale position is along a line 55, 56 shown in Fig.3 corresponding to the 270 degree point of Fig. 8.

The angular direction of the pull-ofi held result? ing from-magnetisminduced in the soft iron pull-off elements 35 or 36 is along the line51, 58 corresponding to the 315 degree point of Fig. 8. 'I'he magneticaxis of the pull-off magnet I3 is alongthe line 59, 60 corresponding tothezero degree point of Fig. 8. It will be understood that in thearrangement of Fig. 3 the magnetic flux emanating from the pull-oil'magnet I3 and acting upon the rotor II will be along a diameter 3o 6I,62 with the lines of flux passing from the end 6I of the diameter to theend 62 which is equivalent to having a permanent magnet placed in theline 6 I 62 with its north-seeking pole toward the end 62 and itssouth-seeking pole toward the end 6I corresponding to the arrangement ofFig. 8.

Although in Fig. 3 I have shown soft iron pull? oi elements in the 'formof screws 35 and 36, it will be understood that my invention is notlimited to this specific arrangement and thesoft viron pull-on eiect maybe obtained from the conformation of the magnetic shield 31 itself, ifthere is any eccentricity or if one side thereof is attened, or if theshield 31 is not perfectly circular. For example, if desired the shield31 may be slightly elliptical with a difference of ooo or A000 of aninch between the major and minor axes in an instrument having a shielddiameter of inch. Thus, the eiect of the softiron elements 35 and 36will 'be obtained :by making the shield 5 empticaiwith its minor axisalong the une s1,

`68 and with the major axis of the ellipse perpendicular-to the line 51,68.

It'will be understood that circular grooves 63 and-64 are provided inthe bottom of the frame :s and in the end surface of the sub-bridge 4oto receive the circular or slightly elliptical shield 31. The widths ofthe grooves 63 and 64 arie made sumcient to permit rotation of theshield 31 in the grooves although it may not be perfectly `50 circular.Thus, angular adjustment of the shield 31 is provided and the shield 31is capable of being secured in the desiredangular position whentheproper adjustment is found by screws 65 or other suitable means whichserves to hold down the 05 cover and sub-bridge 40 against the top edgeof the magnetic shield 31. In fact, I have found that frequentlymanufacturing virregularities in the case of shields intended to becircular cylinders are suiiicient to provide soft iron pull-off torqueeffect. Itis highly important in the case of such manufacturingirregularities to rotate the shield 31 to the proper angular positionand to secure it there. Otherwise the pull-off eilect in the scale rangeof the instrument may correspond to that shown at the 90 degree point inFig. 8 or the peak 3B of the resultant curve 29 in Fig. 9. The use ofelements such as screws 35 or 36 may be delforegoing; types ofinstruments.

sirable for strength adjustment even when the l shield 31 serves as s,soft iron pull-on element.

Although Iv have referred to the pull-olf effect of the lowcoerciveforce magnetizable elements I1, 35, 36 or 31 as the soft iron effect, itwill be understood that the pull-on element or elements which are notpermanently magnetized may be composed of any suitable relativelypermeablemagnetic material. In the case ofthe magnetic shield 31, forexample, satisfactory results are obtained by utilizing an alloy soldunder the name of Mu-metal."

My invention is not limited tothe employment of the combinationpermanent-magnet, soft-iron pull-off arrangement in indicators of thespecic type` disclosed in Figs. 1 to 4. Fig. 5 shows an instrumentcomprising a cylindrical or slightly elliptical magnetic shield 31enclosing a pair of angularly mounted field coils 61 having radialmagnetic axes at an angle of about 120 degreesv intersecting the axis ofrotation I2 of a rotor, which may in this case consist of a cylindricalmagnet 68 magnetized transversely to its axis of rotation. A pull-offmagnet I3 is mounted at a suitable point within the magnetic shield 31and soft-iron pull-off elements I1 are also mounted -within the magneticshield 31. As exl. plained in connection with Figs. l to 4 the softironpull-0E eiect may be obtained from the eccentricity of the shield 31instead of employing separate elements I1.

With a uniform pull-oft torque throughout the lengthv of .the scaleangle of the instrument, it

olf field is to deflect the pointer'slightly in the down scale directionfrom the position it would have if there were no pull-off field. Thiseffect may be taken care of by shifting the pointer on the rotor orshifting the scale slightly. If anon-uniform pull-olf eld were employed,it

' would be necessary to calibrate the scale and a different calibrationwould be required foreach different type of instrument. 'This wouldpreclude the use of standard printed scales with all the instruments ofa given type. Consequently, the use of my uniform lpull-ofi torque.arrange- `ment has the advantage .of making it vunnecessary torecalibrate the 'scale of the instrument when a pull-o feature isprovided in the instrument. It also has the advantageof permitting fixedscales to be used with all instruments of a given .type whether or notthe pull-oli feature is to be utilized and whether or not the instrumentd.

or the system in which it is employed is such as to produce a linearscale calibration.

, Another importantv advantage of having the pull-0E torque uniform isthat itpermits lutia weaker pull-off than that which would yotherwise berequired. With a non-uniform field the pull-off effect is usually a.minimum near the zero end of the scale. It is therefore necessary t makethe pull-off eld strong enough to move the pointer olI scale from thezero position. The result is .that excessively strong pull-olf afieldsare required in other regions of the scale.

Accordingly the use of a uniform pull-off field minimizes voltage andtemperaturev errors because it is not necessary to provide so. muchtorque orhave so much wire dissipating heat in the field windings of theinstrument.

- Temperature compensation may, however, be employed in combination withthe pull-off arrangement described in connection with lthe ,will beapparent that the only effect of the pull- Y For example, the soft-ironpull-oil elements I1 may be com, posed of blocks of permeable magneticmaterial having a permeability which varies in response to temperature.For instance, one or more of such blocks may be mounted at the innersurface of the magnetic shield 31 at the position I1 represented in Fig.5 in the manner described in connection with Fig. lof my copendingapplication, Serial No. 466,436. Such temperature compensating elementsmay be composed of an alloy known as carpenters steel,

or of the alloy described in Patent No. 1,706,172,

Kinnard, which consists of to 20 per cent copper, 60`to 80 per centnickel, and including about 2 per cent iron.

I, have herein shown and particularly described certain embodiments ofmy invention and certain methods o f operation embraced therein f or thepurpose of explaining its practice andfshowing its application, but itwill be obvious to those skilled in the art that many modifications andvariations are possible,v and I aim therefore to cover all suchmodifications and variations as fall within the scope of myinventionwhich are defined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A pull-off arrangement for a moving magnet instrument having a.scale, one end of which is referred to asthe zero end of the scale,saidarrangement comprising.r in combination with the movable magnet ofthe instrument a permanent magnet mounted in such a position as toattract the movable magnet of the linstrument to a position beyond thatcorresponding to -the zero end of the scale and a magnetizable elementcomposed of low coercive force magnetic material mounted in such aposition as to attract the movable magnet of the instrument stillfurther beyond the said zero scale end position. tne relative strengthsof the magnets and the dimensions of the magnetizable element beingchosen to make the maximum reaction between composed of low coerciveforce magnetic material mounted in such a position as to attract themovable magnet'of the instrument still further beyond the said zeroscale end position. 3. A pull-olf arrangement for a moving magnetinstrument having a scalefone end ofv which is referred to as the zeroend of the scale, saidv arrangement comprising in combination with themovable magnet of the instrument, which has a position corresponding tothe zero end of the scale, a permanent magnet and a magnetizable elementcomposed of low coercive force magnetic material, each of the two latterele- I ments being mountedin such a position as to attract the movablemagnet beyond its zero scale end position.

, and having 4. A pull-of! arrangement for a denecting ,pointerinstrument having a scale one end ot `which is referred to as the zeroend oi the scale a movable magnet one position of which corresponds tothe zero end of the scale, said arrangement comprising in combinationwith the movable magnet a permanent magnet mounted in a position todeflect the movable magnet in a direction toward the zero end of thescale, and a magnetizable element composed of low coercive forcemagnetic material mounted to deflect the movable magnet further in thedirection toward which it is deected by the permanent magnet when nearthe zero position.

5. A pull-01T arrangement for a4 derlecting pointer instrument having' ascale one end of which is reierred to as the zero end of the scale andhaving a movable magnet, one position of which corresponds to the zeroend of the scale, said arrangement comprising in combination with such amovable magnet a stationary permanent magnet mounted in a position todeect the movable magnet in the down-scale direction and a stationarymagnetizable element composed of low coercive force material mounted insuch a position as to deflect the movable magnet in a down scaledirection when the movable magnet is near the zero scale end position,at least one oi said stationary elements being positioned to deiiect themovable magnet in the down-scale direction beyond its zero scale endposition.

6. A pull-od arrangement for a deilecting pointer instrument having ascale one end of which is referred to as the zero end or the scale andcomprising iirst and second relatively movable members, one 'of therelative positions of which corresponds to the zero end of the scale,the first of said relatively movable members comprising a magnet and thesecond of said relatively movable members comprising a plurality ofelements. one of which constitutes an element for reacting withthe saidmagnet and producing relative movement in the up-scale direction betweensaid relatively movable members, said pull-off arrangement comprising incombination with said magnet a pair of pull-oil elements mounted on thesecond relatively movable member, the first of said elements comprisinga pulloi magnet and the second comprising a magnet-` izable elementcomposed of low coercive torce magnetic material, said pull-off elementseach being mounted in such a position on the second relatively movablemember as to tend to produce relative motion between said members in thedown-scale direction.

FREDERICK R. SIAS.

